Devices for measuring the warp tension in weaving looms are necessary, mainly for two purposes. In connection with the closed loop control of the warp tension the measurement is required to assure always an advantageous tension on the warp threads with due regard to the type of fabric to be produced. The measurements are also necessary to make sure that rupture of the warp threads is avoided.
It is known to ascertain the warp thread tension by way of a pressure measurement, whereby a pressure sensitive hose is arranged across the warp thread direction, whereby the pressure sensitive hose lies between the warp threads in such a manner that a portion of the warp threads is guided around the top surface of the pressure sensitive hose while another portion of the warp threads is guided around the underside of the pressure sensitive hose. The tension on the warp threads applies pressure to the pressure sensitive hose and such pressure is approximately proportional to the tension on the warp threads. The so ascertained pressure or tension of the warp threads may be indicated or it may be used in connection with a measuring device for a steady closed loop control or follow-up control of the tension on the warp threads.
However, the compression ratios or values ascertained as just described are subject to the disadvantage that they depend on the looping angle of the warp threads around the sensor hose. Additionally, the so ascertained values depend on the prevailing temperature so that the warp thread tension is ascertained only relatively inaccurately not to mention possible impairments of the warp threads.
Another known method for measuring the warp thread tension is directed to measuring the force which is applied by the warp threads to the backrest or back rail. The backrest or back rail serves in a known manner as a guide means for the warp threads which thus exert a certain compression force on the back rest depending on the tension of the warp threads.
It is a disadvantage of the just mentioned measuring method that it also depends on the looping angle of the warp threads around the backrest with the further complication that this looping angle changes continuously during the weaving due to the fact that the warp threads are taken off from the warp beam. As a result, a substantial measuring error may occur.
Another known apparatus for measuring the warp thread tension comprises means for guiding one or several warp threads along a meandering path formed between three measuring rollers. In this manner it is possible to measure the pressure that is exerted by the warp thread on the measuring rollers which are deflected by such pressure. A disadvantage of this type of measuring resides in the fact that the increased friction of the warp thread or threads within the meandering measuring path may influence the measurement. Another disadvantage is seen in that the warp thread passing through the tension measuring rollers may break due to the increased friction, thereby causing a time consuming repair of the broken warp thread and a reinsertion into the measuring device.